Semiconductor lasers have been widely used in printers, optical disks, optical communications and the like because of their compact size, lower cost, and easiness of obtaining a laser light simply by passing a current. However, the current-output power (optical amount) characteristic of the semiconductor lasers varies depending on the environmental temperature. Because of this feature, it is required to perform a special output power control to obtain a fixed (constant) optical output power. The power (optical output power) control is called Automatic Power Control (APC).
In the Automatic Power Control (APC), before a semiconductor laser is practically used, the semiconductor laser is operated so that the output power of the semiconductor laser is received by using a photodiode (FD). Then, a current value when the output of the photodiode (FD) reaches a predetermined level is stored in a storage means. By using the current value, the output power is controlled to obtain a stable output power. With the recent increase of writing speed in printers and the like, a method has become popular in which plural semiconductor lasers disposed in an array shape are simultaneously driven. In such a semiconductor laser array, the photodiode(s) to be used for the Automatic Power Control (APC) is integrated without fail. However, the number of the photodiodes may be smaller than that of the semiconductor lasers; and in some cases, only one photodiode is integrated.
When the number of the photodiode (FD) is only one, it is required to separately drive the semiconductors one by one. FIG. 7 is a block diagram showing a prior-art relationship among a multi-beam laser power control circuit 15, a semiconductor laser array (multi-beam unit) 5, and an image control circuit 54. As shown in FIG. 7, the semiconductor laser array (multi-beam unit) 5 includes two semiconductor lasers (LD1, LD2) and only one photodiode 3 corresponding to each of the semiconductor lasers (LD1, LD2).
In the semiconductor laser array (multi-beam unit) 5 of FIG. 7, if the Automatic Power Control (APC) is simultaneously performed on those two semiconductor lasers (LD1, LD2), the photodiode 3 regards the total output power from the two semiconductor lasers (LD1, LD2) as the basic output power of each of the two semiconductor lasers (LD1, LD2). Namely, in this case, the APC may be terminated before the power output from the two semiconductor lasers (LD1, LD2) becomes sufficient.
To address such an inconvenience, conventionally, for example, there is a known timing chart in which a time margin is inserted between plural timings at which corresponding APC execution signals (APC1, APC2) are input into the multi-beam laser power control circuit 15, the time margin being determined so as to prevent the APC execution signals (APC1, APC2) from being overlapped with each other when the APC execution signals (APC1, APC2) are input into the multi-beam laser power control circuit 15 in view of parasitic resistance/capacitance due to substrate wirings and the like, the APC execution signals (APC1, APC2) corresponding to two semiconductor lasers and being output from the image control circuit 54 that is provided in a preceding stage of the multi-beam laser power control circuit 15 and that controls the multi-beam laser power control circuit 15 (see, for example, a timing chart of FIG. 8 of Patent Document 1, a timing chart of FIG. 9 of Patent Document 2, and a timing chart of FIG. 8 of Patent Document 3).